A form should feel easy to use. The visitor sees a question, gives an answer, and needs to know what will happen next.
What You Need to Know
The :valid and :invalid selectors respond to form constraint states without requiring a validation library.
In this example: style an invalid required field.
Try the Example
Take a look at the example. We’ll explain the important lines right below it.
<form action="/join" method="post">
<label for="email">Email</label>
<input id="email" name="email" type="email" required>
<button type="submit">Join</button>
</form>
<style>
input:invalid { border: 2px solid crimson; }
input:valid { border: 2px solid seagreen; }
</style>Let’s Read the Code
- <form>: The form groups controls and identifies where submitted data goes.
- <label>: The label tells people what the input is for. A matching for/id pair makes the label clickable.
- for: For connects a label or output with a control by ID.
- id: An id uniquely identifies an element on this page.
Read the named elements and attributes together. They explain how the browser connects this feature to the rest of the page.
What Happens in the Browser?
The HTML controls work in the browser, but saving an answer requires the example action path to be handled by a real server.
How can you test it? Submit once with an empty field, once with a clearly invalid value, and once with a valid value. Watch which rule blocks submission. These are helpful browser checks; your server must validate all received data again.
Your Turn
Try an invalid email and a valid email and compare the border styling.
Common Beginner Mistakes
- Copying the example without giving the fields a clear label and a usable name.
- Color is not enough on its own to explain an error. Add readable messages when a check fails.
Key Takeaways
- With validation Pseudo-Classes and User Feedback, the main idea is to match the HTML to the visitor’s task, not just to copy an unfamiliar attribute.
- <form>: The form groups controls and identifies where submitted data goes.
- Remember that HTML form rules help the visitor, but server-side code must decide whether received data is valid.